Differences between class 1, class 2 and class 3 RNR (Ribonucleotide Reductase)

CLASS 1 RNR:

1. Present in prokaryotes and eukaryotes.
2. Substrate - NDP (Nucleoside diphosphate)
3. Cofactor = Ox bridged binuclear iron center
4. Reductant- Glutaredoxin  and thioredoxin
5. Conditions = functional under aerobic conditions, requires O2 for activation
6. Method of generation of free radicals = the binuclear Fe3+ center interacts with Tyr122 to form the tyrosyl free radical resulting in the generation of a thiol radical (free radical of cys)
7. Exists as alpha2beta2 or alpha2beta6 oligomers.

CLASS 2 RNR:

1. Present in prokaryotes.
2. Substrate = NDP
3. Cofactor= 5-deoxy adenosyl cobalamin
4. Reductant = Glutaredoxin and thioredoxin
5. It is oxygen independent. Can function under both conditions.
6. Radical is generated by homolytic cleavage of the 5-deoxy-adenosyl-cobalamin cofactor. C-Co (III) bond generating a 5-deoxy-adenosyl radical - used to generate thiol radical.
7. Exists only in alpha2beta2 state.

CLASS 3 RNR:

1. Present in anaerobic prokaryotes.
2. Substrate = NTP (nucleoside triphosphate) 
3. Cofactor = [4Fe- 4S] cluster and requires SAM and NADPH for activity
4. Reductant = Provided by oxidation of formate to CO2
5. It is only functional under anaerobic conditions and sensitive to O2.
6. Generated by NADPH supplied and Fe-S cluster mediated one electron reductive cleavage of SAM to yield the 5-deoxy-adenosyl radical which then generates a stable glycyl radical (free radical of glycine , O2 sensitive radical).
7. exists in alpha2 + beta2